This application claims the priority of German applications 198 31 508.2 and 199 11 333.5 filed in Germany on Jul. 14, 1998, and Mar. 15, 1999, respectively, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to an arrangement for condensing a drafted fiber strand in a condensing zone arranged downstream of a front roller pair of a drafting arrangement, which condensing zone comprises a sliding surface having a suction slit extending in essentially a fiber strand transport direction, and which condensing zone also comprises a perforated transport belt which transports the fiber strand over the sliding surface.
In the case of a device of this type (U.S. Pat. No. 5,600,872), the transport belt is made in the way of a drafting arrangement belt, but consists, however, of a material which has a greater elasticity. The greater elasticity can be achieved in that, for example, the transport belt used in the condensing zone has no fabric inserts. The transport belt comprises, centrical to the transport direction, perforations whose size is dependent upon the yarn number of said fiber strand and which determine the condensing of the drafted fiber strand. The perforations therefore determine the width to which the fiber strand is to be condensed.
The geometry of the known transport belt inevitably means that there is a certain distance between the fiber strand to be condensed and the suction slit. The perforations of the transport belt also cannot measure less than a certain minimum size, and there has to be a minimum distance between two perforations. Overall, the result is a not very homogeneous suction, which furthermore, in particular due to the inevitably occurring inleaked air, requires a relatively strong vacuum source.
It is an object of the present invention to ensure the best possible homogeneous suction in the condensing zone, without an all too high vacuum being necessary.
This object has been achieved in accordance with the present invention in that the transport belt is made of such a thin material that the fiber strand is disposed on the sliding surface and the suction slit at practically no distance thereto.
A very finely perforated woven, knitted or warp knit material can be used for the transport belt. Thin, perforated foils made of plastic or metal are also contemplated.
In contrast to the above described prior art, in the case of the present invention it is the suction slit located under the transport belt which determines the degree of condensing, rather than the size of the perforations. The perforations of the transport belt, which can be very fine and close meshed in a fabric, ensure a very even and thus homogeneous airflow. Because the transport belt, due to the thin material, is disposed on the sliding surface and the suction slit at practically no distance thereto, all the air which is sucked in becomes practically "effective air", which results in a particularly good degree of effectiveness in condensing. Practically no air is sucked in which is not used effectively during condensing.
A woven fabric comprising polyamide multifilament yarns having a diameter of less than 0.1 mm, for example 0.08 mm, has proven to be suitable for the purpose of the present invention. Despite this very fine fabric, it is stiff enough for an edge guide due to the synthetic multifilaments.
The inside diameter of the openings of the perforation should measure at the most one tenth of the width of the suction slit. It has been shown that the finer the perforation, the more even the suction is. In the case of a foil it is sufficient when the transport belt is perforated only in the area of the suction slit, whereby the perforation should, however, be somewhat wider than the suction slit. In the case of woven, knitted or warp knit materials, the perforation can also be essentially limited to the area of the suction slit, whereby here also the air permeable area should be wider than the suction slit. The covering of fabric openings located laterally adjacent to the suction slit could be effected by a chemical finish.
In the case of warp knit or knitted materials, the inner diameter is taken to be the mesh width, in the case of woven fabrics the distance between adjacent warp fibers and adjacent weft fibers. In order to increase the air permeability in the area of the suction slit in the case of a woven fabric, it can be provided that in this area the number of warp fibers are reduced.
Although, as mentioned above, the fine perforations are desirable for technical reasons, they can result in practice in blockaging and thus to a reduction in air permeability. It can, therefore, for purely practical reasons, be necessary to make the openings larger than is technically advantageous, for example larger than the width of the fibers of the fiber material to be processed, so that any possible remaining fibers, or the like, are sucked through the openings thus avoiding blockages.
In certain preferred embodiments of the present invention, the suction slit extends slightly diagonally to the transport direction, whereby its width lies in the order of magnitude of 1.5 mm. The suction slit is thus wider than the ready condensed fiber strand, yet somewhat narrower than the not yet condensed fiber strand. The suction slit is best disposed at an angle of between 18 and 20.degree. diagonally to the transport direction, which gives rise to slight false twist in the fiber strand to be condensed. The fiber strand hereby follows the direction of the suction slit and is falsely twisted by the transport belt, whereby the edge fibers are rolled inwards. This results in the desired reduced hairiness and at the same time to a better material utilization and thus to an increased tear resistance.
In certain preferred embodiments of the present invention, the transport belt is driven by a nipping roller, which presses the transport belt onto the sliding surface, whereby the suction slit extends to the nipping roller. The latter is particularly important in order to prevent the condensed fiber strand becoming undone before the fiber strand reaches the nipping gap. From the nipping line onwards begins the effect of the spinning twist, in relation to which the nipping roller forms a twist stop.
The disposition of the transport belt is then particularly close when the sliding surface is curved. This prevents the transport belt from rising occasionally from the sliding surface.
The transport belt forms advantageously an endless woven loop, whereby any points of discontinuity of the transport belt in transport direction are avoided. Points of discontinuity can result later in the undesirable moire effect when the spun yarn is woven.